CAUSE: Blistering refers to the formation of a blister caused by vapor expanding between the coated surface and the film. Due to humidity, construction defects, or lack of effective vapor barriers, moisture condenses on or saturates wood siding. Lower humidity and high temperature generate pressures within wood and on adhesive bond of the coating as trapped moisture evaporates. Solvent based coatings are most susceptible. Fresh blisters are filled with water. As the surface dries, water evaporates, but the paint film does not recover. Blistering is accelerated by the swelling effect of moisture on wood. Continued swelling and contraction between day and night humidity and temperature variations weakens adhesion. Another cause is solvent entrapment, or painting with solvent based coatings in direct sunlight on a surface that is too warm. This problem is prevalent in darker colors, due to absorption of heat, more than light colors. Thicker application or recoating prior to complete cure of a previous coat contribute to this type of failure. Under this condition, the paint surface dries before all the solvent is released from the underlying film. Expansion of trapped solvent produces blisters.

SOLUTION: If blistering was caused by moisture, a number of solutions exist. Repair loose caulking and install vents or exhaust fans. If the home has lap siding, install siding vents. Vents or fans allow moisture from in the house to evaporate before penetrating wood siding. This will minimize future blistering problems. Sand and scrape peeling paint to bare wood. Prime exposed areas and repaint. If large areas of paint have blistered and need to be removed, high pressure washing or use of a heat gun will speed the process. Heat blisters develop in FRESHLY applied coatings only. When broken, they don't contain liquid and are not associated with a surface defect that requires remediation. These blisters may be removed by scraping and sanding. The surface should be recoated under more favorable conditions.

Above article reprinted by the kind permission of Benjamin Moore Paints.